Because banknotes and similar documents are theft-attractive, protective storage devices have been developed so as to keep them in a protective casing. Staining (also referred in the art as neutralizing) of banknotes is a security measure occurring due to the activation of anti-theft devices equipped in cash-in-transit (CIT) vans, automatic teller machines (ATMs), security cases, safes or safety boxes. For example, banknotes stored or stacked in an enclosure or banknotes transported in bundles are placed into closed and locked containers which also contain a cartridge with a specific ink. Upon any unauthorized attempt or breach to open the container, a specific ink, often called staining ink (also referred in the art as indelible ink), will be ejected or delivered. In case of an unauthorized attempt or breach of security, the staining ink comes in contact with the banknotes, thus allowing the migration of the ink on and between the banknotes stored and stacked or piled in the enclosure, thereby staining them and permanently and irreversibly marking them by obtaining a typical, well visible or recognizable change.
Known existing alternatives to staining comprise chemical and/or mechanical ways including for example the perforation or mutilation of the banknotes. However, such systems are complex and require complicated machinery and a fair amount of power. Chemical solutions have been devised and include without limitation smoke generators (e.g., smoke dyes) or strong acids such as chlorosulfonic acid. However, smoke dyes are not highly robust and strong acids are likely to impair health and environment.
Consequently, staining inks are commonly used as a protection of banknotes against robbery or theft and are used for invalidating them by permanently damaging or defacing by staining those that have been stolen and, which are thus unusable and unfit for circulation. Permanently ink-stained security documents are ideally easily perceived by the naked eye.
On another hand, with the aim of increasing the durability and cleanliness and thus the circulation lifetime of security documents, in particular banknotes, it has been a conventional practice to protect the security document with a protective varnish so as to provide a protective varnished document. In addition to the increased durability through soiling or chemical resistance, protecting the surface of a banknote simultaneously increases the durability and resistance of overt (i.e. visible to the unaided human eye) and covert (i.e. visible or detectable only with the help of an instrument) security features.
Unfortunately, however, varnished security documents may suffer from a poor ability to retain the staining ink. Should the varnished security document retain a small amount of the staining ink due to small cracks, pinhole spots, or defects of the protective varnish arising upon exposure to external conditions, the quantity of said retained ink would nevertheless be not significant enough to be easily detected by visual human authentication, would be easily confused with normal wear, and might therefore be wrongly kept in circulation.
There remains a need for an efficient and universal technology to make varnished security documents, in particular banknotes, sensitive to staining ink in case of an illegal process, said technology being suitable for any anti-theft machines and any staining ink. Therefore, a need remains for varnished security documents, in particular banknotes, exhibiting a satisfactory staining retention and/or adsorption while maintaining good resistance to chemicals so as to remain permanently damaged or defaced, and thus be unusable and unfit for circulation.